i doubt there's any chance it'll be CK2; they're actively developing it and it's still profitable.
a big part of me hopes that it'll be vic3, as it's my favourite of the grand strategies, but i think i'll be disappointed.
certainly a grand strategy about rome could be profitable. there hasn't been anything since rome total war 2, and given what a disappointment that turned out to be, i think that could be quite profitable for paradox. if it's done to the same high standard as their other recent games have been on release, then i think it could be especially popular. i wonder whether they would base it on the EU4 model/engine, or make a new one?